Conventional rods for rock drilling either have a thread machined directly in the rod or a thread machined in a rod end forged to a diameter larger than the rod diameter. Instead of forging up the dimension of rods it is possible to friction weld end pieces or guiding pieces with diameters larger than the rod diameter. Conventional rock drilling rods are most often manufactured from holed rods and adapters from solid rods. For threaded rods at least one thread is often machined in a bumped up (forged) end with a diameter larger than the rod diameter while shank adapters often are machined from rounds.
Instead of forging up the dimension or using extensive machining it has been more and more common to friction weld together pieces with large differences in diameter. However, drill equipment manufactured from conventional drill steel in this manner include weak, i.e., "soft", heat affected zones (Z) on both sides of the weld interface, as shown in FIGS. 1A and 1B. These soft zones (Z) have lower hardness than the unaffected portions of the rod and are thus the weakest parts of the rod. To increase the strength of the soft zones to approach that of the unaffected portions of the rod, it is therefore necessary to alternately carburize, normalize or harden this type of rod after friction welding, as shown in FIGS. 2A and 2B.
Another way is to compensate for the weaker strength in the soft zones by bumping up the rod end before friction welding, as shown in FIG. 3.